As owner of ns_imap i can say that it in production-quality for about 90-99%. There are some issues with fatal error handling because of the way UW IMAP c-client handles them, but they are very rare. We use it our Webmail based on this module for almost year now without any problems.
More complete Webmail solution is now available in opensource package(
ftp://ftp.crystalballinc.com/pub/vlad/oss2.tar.gz) and it does many features of IMP, not everything because we did not need all of them.
Before it we used NeoMail, IMP, SquirrelMail and others, but we wanted same environment(including GUI) where all tools can be easily connected to each other, reuse same database and data, easily configured and maintained. If everything runs in AOLserver it may be pretty easy. Supporting apache/php and other tools can be easy too, but they are additional components.
There is one more product which looks interesting, dbMail(www.dbmail.org), they store email in the database and building webmail client is just writing/reading from database, they have couple webmail clients, writing in OACS may be very easy task.
